The present invention is related to a method and system for generating a reference voltage. More particularly, the present invention is related to a CMOS bandgap reference voltage circuit that operates on low-voltage power supplies with low power consumption.
Bandgap voltage references are used as voltage references in electronic systems. The energy bandgap of Silicon is on the order of 1.2 V, and is independent from temperature and power supply variations. Bipolar transistors have a negative temperature drift with respect to their base-emitter voltage (Vbe decreases as operating temperature increases on the order of xe2x88x922 mV/deg C.). However, the thermal voltage of a bipolar transistor has a positive temperature drift (Vt=kT/q, thus Vt increases as temperature increases). The positive temperature drift in the thermal voltage (Vt) may be arranged to compensate the negative temperature drift in the bipolar transistor""s base-emitter voltage. Bandgap reference circuits use the inherent characteristics of bipolar transistors to compensate for temperature effects and provide a stable operating voltage over various power supply and temperature ranges.
One example bandgap reference circuit includes two bipolar transistors that are arranged with a common base. Two resistors are series connected between the emitter of the first bipolar transistor and a common ground. The emitter of second bipolar transistor is connected to the common point between the two resistors. The two bipolar transistors are arranged to provide a ten-to-one (10:1) current density difference with respect to one another. The ten-to-one current density results in a 60 mV difference between the base-emitter voltages of two bipolar transistors (xcex94Vbe=Vt*ln(I1/I2)=26 mV*ln(10)=60 mV, at room temperature). The 60 mV difference appears across the first resistor. The voltage between the base of the bipolar transistors and the ground terminal provides a voltage reference (VREF) that is roughly given as VREF=Vbe+X*Vt, where X is a constant that is used to scale the temperature correction factor. The temperature correction factor (X) is adjusted by the ratio of the resistors. Typical temperature corrected reference voltages of 1.25 V are achieved by this configuration.
The present invention is related to a voltage reference that operates from a low power supply voltage. More specifically the present invention is directed to voltage references that operate in the power supply range from approximately 1.2 volts to 6 volts. The bandgap reference is implemented in a CMOS process such that smaller die sizes and power consumption may be decreased.
Briefly stated, a voltage reference circuit is arranged in a CMOS process based technology to provide a configurable voltage reference. The voltage reference includes bipolar transistors that are implemented as parasitic devices in the CMOS process. Two of the bipolar transistors are configured to generate a xcex94Vbe signal in the voltage reference circuit. An error amplifier cooperates with the two bipolar transistors via a control signal such that the control signal is related to xcex94Vbe/R. A first current source is coupled to another bipolar device, which is parallel connected to a resistor divider. The output of the resistor divider provides a divided reference signal that is related to the Vbe of the other bipolar device. Another resistor is coupled between a second current source and the output of the resistor divider such that an adjustable/temperature compensated reference signal is provided.